1. Field of the Invention
The invention relates to a pilot valve for applying or removing a control fluid pressure to a fluid pressure actuator in response to variations of a monitored fluid pressure either above or below a desired normal value.
2. Background of the Invention
Pilot valves for sensing a control fluid pressure and shifting a valve element in response thereto have been widely employed in the prior art. A common form of such valve is the so-called "block and bleed" type which commonly employs a pressure balanced spool valve which is shiftable, responsive to a change in a monitored fluid pressure, to apply or exhaust a control pressure to an actuator which is controllably related to the monitored fluid pressure. The spool valve is normally urged to one position by a spring whose force is opposed by the monitored fluid pressure to be controlled acting on a piston operatively connected to the spool valve to urge the spool valve towards a second position. See, for example, U.S. Pat. No. 4,420,011 to ROGER.
The prior art pilot valves of the "block and bleed" type have uniformly incorporated a pressure balanced spool, i.e., the spool valve is not subject to any pressure forces derived from the control fluid which passes through the valve body in response to a shift in axial position of the spool valve. A balanced spool valve is subject to the practical disadvantage that a wider swing of the control of the monitored fluid pressure must be tolerated due to the friction and inertia forces involved in initiating the movement of the spool valve in response to a change in the monitored fluid pressure from the desired normal value. Thus, when the value of the monitored fluid pressure exceeds the desired norm by an amount sufficient to overcome the friction and inertia forces inherently present in any spool valve, it necessarily follows that the spool valve will not be restored to its original position until the monitored fluid pressure swings in the other direction an equal amount beyond the desired norm. Thus, the sensitivity of the pilot valve is substantially diminished and the accuracy of control of the fluid pressure being monitored is substantially impaired.